Method of fabricating semiconductor thin film and method of fabrication Hall-effect device

ABSTRACT

A method of fabricating a semiconductor thin film is initiated with preparing a substrate having a surface consisting of a single crystal of Si. The surface has an oxide film. Then, the oxide film is removed. The dangling bonds of the Si atoms on the surface are terminated with hydrogen atoms. An initial layer is formed on the substrate of the single crystal of Si terminated with the hydrogen atoms, of at least one selected from the group consisting of Al, Ga, and In. A buffer layer containing at least In and Sb is formed on the initial layer. A semiconductor thin film containing at least In and Sb is formed on the buffer layer at a temperature higher than the temperature at which the buffer layer is started to be formed. There is also disclosed a method of fabricating a Hall-effect device. This method is initiated with forming a semiconductor thin film by making use of the above-described fabrication method. Then, electrodes are attached to the thin film.

This is a continuation of application Ser. No 08/247,655 filed on May23, 1994, now U.S. Pat. No. 5,385,864 which application is entirelyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating a semiconductorthin film adapted to be used as a Hall-effect device which is used todetect rotations, displacements, and other motions.

2. Description of the Related Art

Hall-effect devices using semiconductors have several advantages. Forexample, they have excellent frequency characteristics, are capable ofnoncontacting-type detection, and are insusceptible to noise. They havebeen used principally as noncontacting-type rotational number-detectingsensors and have found wide application. Among them, a Hall-effectdevice using indium antimonide (InSb) which is a semiconductor havingthe greatest electron mobility produces a large output signal. Moreover,a wide gap can be secured between this Hall-effect device and a sampleto be investigated. Hence, this device is most suited for a rotationalnumber-detecting and displacement-detecting sensor. Hall-effect devicesusing InSb include magneto-resistors and Hall generators. The prior arttechniques are described below in further detail, using themagneto-resistors.

Conventional InSb magneto-resistors are classified into the bulk typeand the thin-film type, according to the fabrication process. The bulktype is fabricated by bonding a single bulk crystal to the top surfaceof a support substrate with adhesive, polishing the crystal into a thinleaf, and machining or processing the leaf. In this structure, theelectron mobility has the greatest values of 5 to 8 m² /V s at roomtemperature because of the use of a single bulk crystal. Also, thisbulk-type structure produces a large output signal. However, it is noteasy to handle this device because the electron mobility depends heavilyon temperature. Another problem is that the InSb thin film cracks due toa difference in coefficient of thermal expansion between the adhesivelayer and the InSb at elevated temperatures. Therefore, the operatingtemperature of the device is restricted to the range from approximately-20° C. to +80° C. In high-temperature applications, for example inautomobile applications where the used temperature range is between -50°C. and +150° C., the device lacks reliability and hence is not used.

On the other hand, the latter thin-film type is fabricated by a vacuumprocess such as vacuum evaporation techniques. In particular, a thinfilm of InSb is formed on a substrate by a vacuum process and then thefilm is machined or processed. The device fabricated by this method isinferior in electron mobility to a single bulk crystal because of grainboundaries and dislocations existing inside the film but thetemperature-dependence is milder and so the thin-film type device can beeasily handled. Furthermore, there is a possibility that reliability issecured in high-temperature applications, since the thin film of InSb isdirectly formed on the substrate. In addition, the thin-film type devicecan be made thinner than the bulk type. This makes it easy to increasethe resistance of the device. In consequence, low electric powerconsumption and miniaturization can be accomplished.

In the thin-film type, however, the used substrate plays an importantrole. As an example, where an In Sn film is formed on a substrate whosesurface is amorphous such as a glass substrate, the obtained film ispolycrystalline. The electron mobility is 2 to 3 m² /V s at best, andthe output signal from this device is small. Fukunaka and others haveobtained an electron mobility comparable to that of a single crystal bythe use of a substrate of cleaved mica (Technical Report of ToyoTsushin-ki, No. 40, (1987)). In this method, however, the adhesivenessbetween an InSb thin film and mica substrate is bad. Thus, it isnecessary to transfer the InSb thin film to another support substratevia an adhesive layer. For this reason, the usable temperature range isrestricted to a range similar to the range for the bulk type. Otherknown techniques utilize molecular beam epitaxy to epitaxially grow anInSb film on a substrate made of CdTe, sapphire, BaF₂, GaAs, or othermaterial. Unfortunately, this substrate is very expensive.

Chyi and others produced a thin film of InSb having an electron mobilityof 3.9 m² /V s on a substrate of a relatively cheap, silicon (Si) singlecrystal by molecular beam epitaxy (J.-I. Chyi et al., Appl. Phys. Lett.,54, 11 (1989)). However, this method needs a manufacturing step wherethe Si surface is maintained above 900° C. under an ultrahigh vacuum(normally below 10⁻⁷ Pa) to remove the oxide film on the surface of theSi. It is not easy to use this step in the manufacturing process. Inthis way, with the thin film type, any method of forming a thin film ofInSb having a high electron mobility directly on a substrate at low costis not available and so the thin-film type has not enjoyed wideacceptance.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method offabricating a semiconductor thin film showing sufficiently highreliability even in high-temperature applications by forming an InSbthin film having an electron mobility comparable to that of the bulktype directly on a substrate at low cost.

It is another object of the invention to provide a method of fabricatinga Hall-effect device showing sufficiently high reliability even inhigh-temperature applications by forming an InSb thin film having anelectron mobility comparable to that of the bulk type directly on asubstrate at low cost.

The above objects are achieved in accordance with the teachings of theinvention by a method of comprising the steps of: preparing a substratehaving a surface consisting of a single crystal of Si; removing an oxidefilm from the surface of the substrate and terminating dangling bonds ofSi atoms on the surface with hydrogen atoms; forming an initial layer onthe substrate of the single crystal of Si terminated with the hydrogenatoms out of at least one selected from the group consisting of aluminum(Al), gallium (Ga), and indium (In); forming a buffer layer containingat least In and Sb on the initial layer; and forming a semiconductorthin film containing at least In and Sb on the buffer layer at atemperature higher than a temperature at which the buffer layer isstarted to be formed.

Preferably, the temperature at which the buffer layer is formed islowered with increasing the layer thickness.

Also, the invention provides a method of fabricating a Hall-effectdevice by machining or processing a semiconductor thin film fabricatedby the method described above and attaching electrodes to the thin film.

In the structure described above, the hydrogen atoms prevent the Sisurface terminated with hydrogen atoms from being oxidized. As a result,the surface is maintained stably. Then, the initial layer consisting ofAl, Ga, or In is formed. This permits the following buffer layer to growinto a smooth and large crystal. Also, the crystal is an epitaxiallygrown film having the same crystallographic orientation as that of theSi substrate. Subsequently, the semiconductor film is started to beformed at the temperature higher than the starting fabricationtemperature of the buffer layer. Consequently, the buffer layermitigates the differences in lattice mismatch and coefficient of thermalexpansion between the substrate and the semiconductor thin film.Moreover, the crystal growth rate of the semiconductor thin film isincreased. Hence, an epitaxially grown thin film of semiconductor isobtained which shows high crystallinity and in which individual crystalsare bonded together. Especially, the lattice mismatch with the substratecan be reduced further by lowering the fabrication temperature of thebuffer layer with increasing the layer thickness and then elevating thefabrication temperature to the temperature at which the semiconductorthin film is formed. This further enhances the crystallinity of thesemiconductor thin film.

As described thus far, a crystal of good quality can be directly formedon the substrate and so the adhesiveness between the crystal and thesubstrate is good . A thin film of semiconductor which exhibitsstability at elevated temperatures and has a high electron mobility canbe readily fabricated at low cost.

Where a semiconductor thin film obtained as described above is used, acharacteristic deterioration which would have been heretofore caused bycracks in the film is prevented. A Hall-effect device havingsufficiently high reliability and exhibiting excellent characteristicswithin the temperature range from -50° C. to +150° C. is provided.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, (a)-(d), are cross-sectional views of a semiconductor thin film,illustrating the successive steps of forming the thin film, which formsExample 1 of the invention;

FIG. 2 is a graph illustrating changes in the substrate temperature inthe manufacturing process of Example 1;

FIG. 3(a) is a diagram illustrating an X-ray diffraction patternobtained from a semiconductor thin film formed on (111) Si wafer byExample 1;

FIG. 3(b) is a diagram illustrating an X-ray diffraction patternobtained from a semiconductor thin film formed on (100) Si wafer byExample 1;

FIG. 4(a) is a schematic perspective view of a magneto-resistorfabricated in Example 1;

FIG. 4(b) is a schematic perspective view of a Hall generator fabricatedin Example 1;

FIG. 5 is a graph illustrating the relations among the thickness of aninitial layer of Example 1, the temperature at which a buffer layer isstarted to be grown, and the film structures of obtained buffer layers;

FIG. 6 is a graph illustrating the relation among the ratio of thenumber of evaporated Sb particles to the number of evaporated Inparticles, the substrate temperature, and the compositions of obtainedsemiconductor thin films;

FIG. 7 is a graph illustrating the relation of the substrate temperaturewhen a semiconductor thin film is formed in Example 1 to the electronmobility of the thin film;

FIG. 8 is a graph illustrating changes in the substrate temperature inthe manufacturing process of Example 2 of the invention;

FIG. 9 is a graph illustrating changes in the substrate temperature in amodification of the manufacturing process of Example 2;

FIG. 10 is a graph illustrating changes in the substrate temperature inthe manufacturing process of Example 3 of the invention;

FIG. 11 is a graph illustrating the relation of the substratetemperature when a semiconductor thin film is formed in Example 3 to theelectron mobility of the thin film;

FIG. 12 is a graph illustrating changes in the substrate temperature ina modification of the manufacturing process of Example 3; and

FIG. 13 is a graph illustrating changes in the substrate temperature inthe manufacturing process of Example 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The manufacturing steps of the present example are successivelydescribed by referring to FIGS. 1 and 2. Referring to FIG. 1(a), asubstrate 1 having a diameter of 3 inches is obtained by cutting the(111) surface of a single crystal of Si having a resistivity exceeding1000 Ω-cm. The substrate 1 of a single crystal of Si was cleaned with anorganic solvent, with an acid, and then with an alkali solvent to removecontaminants such as organic materials and metals from the surface.Thereafter, the substrate was immersed in a 5% HF solution for 1 minuteto remove the oxide film on the surface , and to terminate the danglingbonds of the Si atoms at the surface with hydrogen atoms. Subsequently,the substrate was rinsed within deionized water for 5 minutes tocontinue the hydrogen termination.

Immediately after the hydrogen termination processing described above,the substrate 1 was placed into vacuum evaporation equipment. Thepressure inside the equipment was maintained below 5×10⁻⁴ Pa. Under thiscondition, the substrate temperature was set to 300° C. When thesubstrate temperature stabilized as indicated by A in FIG. 2, In wasevaporated by resistance heating to form an initial layer 2 out of In asshown in FIG. 1(b). The evaporation rate was 0.05 nm/s. The thicknesswas 0.2 nm. The pressure was below 1×10⁻³ Pa. This degree of vacuum wasmaintained also in the following steps.

Then, a buffer layer 3 was formed out of InSb on the initial layer 2 bycoevaporation of In and Sb as shown in FIG. 1(c). At this time, thesubstrate temperature was maintained at 300° C. as indicated by B inFIG. 2. The rate at which In was evaporated was 0.1 nm/s. The ratio ofthe number of the evaporated Sb particles to the number of evaporated Inparticles (hereinafter often referred to as the Sb/In) was kept at 1.5.Both In and Sb were evaporated for 200 seconds. Then, the substratetemperature was elevated to 430° C. as indicated by B-C in FIG. 2, andthis temperature was maintained. Thereafter, a semiconductor thin film 4shown in FIG. 1(d) was formed as indicated by C in FIG. 2. At this time,the evaporation rate of In was 0.75 nm/s. The Sb/In was maintained at 2.The In and Sb were evaporated for 2000 seconds.

During the formation of the semiconductor thin film by the stepsdescribed thus far, in-situ observations of the film were made insidethe vacuum evaporation equipment by reflection high-energy electrondiffraction (RHEED). We have confirmed that InSb in the buffer layer 3and in the semiconductor thin film 4 was epitaxially grown on (111)surface of the Si substrate 1. The film thickness of the semiconductorthin film 4 obtained in this way was 4 μm. The crystallinity of the thinfilm 4 was evaluated by x-ray diffraction(XRD). The results are shown inFIG. 3(a). For comparison, InSb (111) surface of a commerciallyavailable single bulk crystal is also shown in FIG. 3(a). As can be seenfrom this graph, the semiconductor thin film 4 showed a diffractionpattern similar to that of the bulk crystal. The electron mobility ofthis thin film 4 was measured by the Van der Pauw's method, and themobility assumed high values of 3.7 to 4.2 m² /V-s at room temperature.The adhesiveness between any adjacent layers of the substrate 1 ofsingle crystal of Si, the initial layer 2, the buffer layer 3, and thesemiconductor thin film 4 was good. A thermal shock test from -50° C. to+150° C. was repeated but any problem such as peeling or characteristicdeterioration did not take place.

Two Hall-effect devices shown in FIG. 4, (a) and (b), respectively, havebeen obtained by photolithographically patterning the semiconductor thinfilm 3 fabricated as described above and attaching electrodes 5. Shownin FIG. (a) is a magneto-resistor. Shown in FIG. 4(b) is a Hallgenerator. The electrodes 5 have been obtained by depositing titaniumand copper in succession by electron beam evaporation and thenphotolithographically patterning these layers. Although a thermal shocktest between -50° C. and +150° C. was repeated for the Hall-effectdevice, any device deterioration such as peeling, cracks, orcharacteristic deteriorations was not observed. We have confirmed thatthe Hall-effect device has quite high reliability.

In this way, in the present example, a semiconductor thin film having ahigh electron mobility and a Hall-effect device having excellentcharacteristics and high reliability can be fabricated.

It is to be noted that during fabrication of the above-describedsemiconductor thin film, the various manufacturing conditions are notlimited to the foregoing. The reason will be described below in greaterdetail about successive manufacturing steps, based on the results ofexperiments.

First, surface treatment of the substrate is described. The Si on thesurface of the substrate 1 after cleaning is oxidized. This oxide filmis amorphous and so the atoms on the surface are arranged irregularly.However, it is known that the oxide film is removed by immersing thesubstrate in the HF solution and that dangling bonds of Si atoms on thesurface are terminated with hydrogen atoms (Hirose, Oyo Buturi , 61, No.11, (1992), p. 1124). If this is rinsed within deionized water, anatomically flat surface is obtained. That is, a monolayer of hydrogenatoms is formed on the surface, and each hydrogen atom is bondedvertically to an inner atom. Furthermore, these hydrogen atoms existstably on the (111) surface, thus preventing surface oxidation. In thisway, a stable surface consisting of a regular array of hydrogen atomssimilarly to a single crystal of Si is obtained. Hydrogen terminationmethods using solutions other than HF solution are also known. Forexample, NH₄ F solution can also be employed. Hydrogen plasma insidevacuum evaporation equipment can be utilized (A. Kishimoto, Jap. J.Appl. Phys. Vol. 29, No. 10 (1990), pp. 2273). Hydrogen ion-beamirradiation can be used. Moreover, heating inside a hydrogen ambient canbe exploited. On surfaces other than (111) surface, the stability ispoor and oxidation progresses within atmosphere. Therefore, hydrogentermination is preferably effected within vacuum evaporation equipment.

The conditions under which various films are formed are describedtogether with the mechanism by which the films are grown. Thecrystallinity of the semiconductor thin film 4 depends heavily on thecrystallinity of the buffer layer 3. The crystallinity of the bufferlayer 3 is affected greatly by the thickness of the initial layer 2 ofIn and also by the temperature at which the buffer layer 3 is started tobe grown. FIG. 5 shows these relations, based on the crystallinity atthe initial stage of formation of the buffer layer 3 where the Sb/In is1.5. In FIG. 5, samples indicated by "◯" have been proved to have beenepitaxially grown. Semi-conductor thin films 4 which were started to begrown within the range indicated by "◯" and formed by the methoddescribed previously had electron mobilities of 3.7 to 4.2 m² /V s. Onthe other hand, each sample indicated by "□" consists of an epitaxiallygrown film mingled with polycrystals. Samples indicated by "Δ" arealigned to the <111>-oriented film (the orientations vertical to thesurface are aligned to <111> but orientations parallel to the surfaceare not uniform). Samples indicated by "X" are polycrystalline films inthe form of clusters. These samples showed electron mobilities less than3 m² /V s. In this manner, where the thickness of In layer is less thanthe thickness of a monoatomic layer on (111) surface of InSb, or 0.1 nm,or where such an In layer is not formed, polycrystals are mingled withan epitaxial layer, thus making it impossible to form the buffer layer 3of high quality. Also, where the In layer thickness is in excess of 2nm, a similar situation occurs. A similar result is obtained where theSb/In is varied. In addition, where the thickness lies within the rangefrom 0.1 to 2 nm, if the substrate temperature is low, only a(111)-oriented film obtained. If the substrate temperature is high , Inparticles coagulate, the buffer layer 3 forms lumps . In consequence, ahigh-quality film cannot be obtained. This temperature range can beextended by increasing the Sb/In at larger values of the In thickness.However, the range could not be made wider than the range obtained wherethe thickness was 0.1 nm. This requires that the thickness of theinitial layer 2 of In be 0.1 to 2 nm and that the temperature at whichthe buffer layer 3 is started to be grown be 250° to 350° C.

Although the InSb layer formed within the temperature range describedabove is an epitaxially grown layer, the range of Sb/In providing thestoichiometric ratios is quite narrow as shown in FIG. 6. It isdifficult to control this range stably. However, an Sb-excessive filmcan have a stoichiometric ratio by temperature elevation becauseexcessive Sb atoms can be released by temperature elevation, since Sbhas a high vapor pressure. At this time, it is necessary that thesubstrate temperature be higher than the temperature at which the vaporpressure of Sb is equal to the pressure inside the evaporationequipment. Specifically, in the present example, the temperature is setabove 370° C. to produce a pressure of 1×10⁻³ Pa. However, if theportions containing excessive Sb are thick, then it is impossible todrive off Sb. Hence, making the film thickness too large is not desired.Since excessive Sb atoms are released, the thickness of the formed InSbfilm is determined by the accumulative thickness of In, i.e.,evaporation rate of In × evaporation time. Experiments have shown thatthe thickness of the InSb film is about 8/3 times as large as theaccumulative thickness of In. We have confirmed that in the Sb-excessiveregion, the result is not affected by the Sb/In. Where this accumulativethickness of In is used, the accumulative thickness of In of the bufferlayer 3 from which excessive Sb atoms can be released is preferably notgreater than about 50 nm. Where the accumulative In layer is thin,coagulation takes place at elevated temperatures and so the accumulativethickness should be greater than 1.5 nm. If the Sb/In is too large, thenthe buffer layer 3 becomes coarse at high temperatures and thecrystallinity deteriorates. Therefore, this ratio is preferably lessthan 6.

During formation of the semiconductor thin film 4, the Sb/In wasmaintained at 2. The relation of the electron mobility of the film tothe substrate temperature under this condition is shown in FIG. 7. Ascan be seen from this graph, good electron mobilities greater than 3 m²/V s were obtained above 370° C. Below 370° C., the composition containsexcessive Sb atoms and, therefore, good films cannot be formed.Preferably, the substrate temperature is set above 400° C., because highelectron mobilities exceeding about 3.5 m² /V s were derived, for thefollowing reason. As the temperature is elevated, the growth rate ofcrystal grains in the direction of the surface is increased, and a goodfilm consisting of bonded crystal grains is formed. Furthermore, theSb/In can be varied within a wide range and so an InSb film of astoichiometric ratio can be obtained. Where the semiconductor thin film4 is formed above 460° C., Sb atoms are released violently from theInSb, thus deteriorating the crystallinity and the surfacecharacteristics. This makes it impossible to have an InSb film of goodquality. Therefore, it is important that the semiconductor thin film 4be formed within the temperature range from 370° to 460° C. Preferably,the temperature range is between 400° and 460° C.

No evaporation rate difference cannot be observed within controllableranges (i.e., the evaporation rate of In is between 0.01 and 1 nm/s, andthe evaporation rate of InSb is 8/3 times as high as the evaporationrate of In) of the present example. The evaporation rates can be set atwill within controllable ranges of thicknesses, taking the evaporationtimes into account. In this way, the semiconductor thin film 4 having ahigh electron mobility can be obtained under various manufacturingconditions as described above. In the present example, the initial layer2 and the buffer layer 3 are formed separately. These layers may beformed successively.

Also in the above example, (111) surface of the single crystal of Si isused as the substrate 1. Where (100) surface is used, a semiconductorthin film that is epitaxially grown in (100) direction is obtained asshown in FIG. 3(b). In the (100) orientation, the electron mobility wascomparable to that obtained in the (111) orientation. In this way, asemiconductor thin film showing good characteristics irrespective of thegrown surface can be obtained.

EXAMPLE 2

The manufacturing steps of the present example are similar to the stepsof Example 1 except for the conditions under which the buffer layer 3 isformed. In Example 1, the substrate temperature is maintained constantduring formation of the buffer layer 3. In the present example, thebuffer layer 3 is formed while elevating the temperature.

The temperature profile in the present example is shown in FIG. 8. Theprocess of Example 1 was carried out until the initial layer 2 wasformed. Then, the buffer layer 3 was started to be grown at 300° C. asindicated by B in FIG. 8. Immediately thereafter, the substratetemperature was started to be elevated. The rate at which thetemperature was elevated was maintained constant such that thetemperature reached 430° C. when the formation of the buffer layer 3ended during this step, the Sb/In was increased from 1.5 to 2. At thistime, the other conditions were the same as used in Example 1.Thereafter, the semiconductor thin film 4 was formed in the same way asin Example 1.

The crystallinity of the semiconductor thin film 4 was evaluated byRHEED and XRD. It was confirmed that the film was grown epitaxially inthe same way as in Example 1. The electron mobilities at roomtemperature were 3.5 to 4.0 m² /V-s, which were close to the valuesobtained in Example 1. Furthermore, the adhesiveness between thesuccessive layers was good. A thermal shock test has shown that thefabricated Hall-effect devices did not deteriorate. Hence, the deviceshave quite high reliability.

The conditions under which the buffer layer 3 was formed is not limitedto the foregoing. This is further described with reference to theprocess of formation of the buffer layer 3. The range of temperatures atwhich the buffer layer 3 is started to be grown is the same as thetemperature range adopted in Example 1. The layer contains excessive Sbatoms. In the present example, the temperature was elevated to 370° C.or above at which Sb atoms are released, while growing the buffer layer3. In this method, Sb atoms can be released by temperature elevation anda stoichiometric ratio can be obtained for the same reason as inExample 1. Since the growth is continued while elevating thetemperature, if the portion containing excessive Sb is thick, then Sbwill not be released. Therefore, the temperature is preferably elevatedto 370° C. or above before the accumulative thickness of In reachesabout 15 nm. At and above 370° C., the Sb/In, the formed film thickness,and the temperature elevation rate can be set at will within the rangein which the stoichiometric composition shown in FIG. 6 can be obtained.

As described above, semiconductor thin films 4 having high electronmobilities can be fabricated under various conditions. In the presentexample, the substrate temperature is increased continuously duringformation of the buffer layer 3. Similar advantages can be derived byelevating the temperature in a stepwise fashion as shown in FIG. 9. Inthe present example, the initial layer 2, the buffer layer 3, and thesemiconductor thin film 4 are formed separately. These layers may beformed successively.

EXAMPLE 3

The manufacturing steps of the present example are similar to the stepsof Example 1 except for the conditions under which the buffer layer 3 isformed. In Example 1, the substrate temperature is maintained constantduring formation of the buffer layer 3. In the present example, thebuffer layer 3 is formed while lowering the temperature.

The temperature profile in the present example is shown in FIG. 10. Theprocess of Example 1 was carried out until the initial layer 2 wasformed. Then, the buffer layer 3 was started to be grown at 300° C. asindicated by B in FIG. 10. Immediately thereafter, the substratetemperature was started to be lowered. The rate at which the temperaturewas lowered was maintained constant such that the temperature reached200° C. when the formation of the buffer layer 3 ended during this step,the Sb/In was maintained at 2. At this time, the other conditions werethe same as used in Example 1. Then, the substrate temperature waselevated to 430° C. at a rate of 2° C./s. Thereafter, the semiconductorthin film 4 was formed in the same way as in Example 1.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.6 to 5.0 m² /V s, which were higher than thevalues obtained in Example 1. Furthermore, the adhesiveness between thesuccessive layers was good similarly to Example 1. The fabricatedHall-effect devices had quite high reliability in the same way as inExample 1.

It is to be understood that the conditions under which the buffer layer3 is fabricated are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation of the buffer layer 3.

The buffer layer 3 was epitaxially grown with excessive Sb in theinitial stage of the growth in the same way as in Example 1. However,observation by RHEED has revealed that as the substrate temperaturedropped., the film changed from an epitaxial film to an amorphous filmcontinuously. The temperature at which the film is amorphized iselevated with increasing the Sb/In. Where the Sb/In is equal to orgreater than 1.5, the film can be made amorphous with certainty bylowering the temperature below 230° C. The film thickness was the sameas the film thickness of Example 1. Under this condition, thetemperature was elevated to the temperature (370°-460° C.) at which thenext semiconductor thin film 4 is formed. As a result, excessive Sbatoms were released, and the upper layer was grown by solid phaseepitaxy in which the lower epitaxial layer acted as a nucleus. Thebuffer layer 3 became smoother than the buffer layer 3 formed inExample 1. At this time, as the temperature elevation rate is increased,the smoothness of the buffer 3 is enhanced and an InSb film of higherquality is obtained. However, because of limitations imposed on theheating mechanism of the equipment, we could not discuss temperatureelevation rates exceeding 3° C./s. On the other hand, where thetemperature elevation rate was less than 0.5° C./s, solid phases grew atrandom inside the upper amorphous layer independent of the solid phaseepitaxial growth from the lower layer. The result is that polycrystalsintermingled. Therefore, it is necessary that the temperature beelevated at a rate exceeding 0.5° C./s until it reaches at least 370° C.

The semiconductor thin film 4 was formed on the buffer layer 3 obtainedin this way, in the same way as in Example 1.

The relation of the electron mobility to the substrate temperature, orthe temperature at which the thin film 4 is grown, is shown in FIG. 11.Semiconductor thin films 4 exhibiting high electron mobilities exceeding3.5 m² /V s over the wide range from 370° to 460° C. were derived.Especially, above 400° C., excellent electron mobilities exceeding 4.0m² /V s were obtained. These films had smooth surfaces. In this way,semiconductor thin films 4 exhibiting high electron mobilities undervarious conditions as described above can be obtained.

In the present example, the temperature at which the buffer layer wasgrown was lowered continuously. As shown in FIG. 12, the temperature maybe lowered in a stepwise fashion. Also, in this case, the surface layerof the buffer layer is amorphized. As the temperature is elevated, theamorphous layer is grown by solid phase epitaxy and becomes high-qualitybuffer layer 3. In the present example, the initial layer 2 and thebuffer layer 3 are formed separately. These layers may be formedsuccessively.

EXAMPLE 4

The manufacturing steps of the present example are similar to the stepsof Example 1 except for the conditions under which the buffer layer 3 isformed. In Example 1, the Sb/In is maintained constant during formationof the buffer layer 3. In the present example, the buffer layer 3 isformed while increasing this ratio.

The temperature profile and the Sb/In profile in the present example areshown in FIG. 13. The initial layer 2 was formed under the sameconditions as in Example 1. Then, the buffer layer 3 was grown at 300°C. as indicated by B in FIG. 13 while increasing the Sb/In at a constantrate with increasing the thickness. The initial value of the Sb/In was2. The value of the Sb/In at the end of the growth was 10. At this time,the other conditions were the same as used in Example 1. Then, thesubstrate temperature was elevated to 430° C. at a rate of 2° C./s inthe same way as in Example 3. Thereafter, the semiconductor thin film 4was formed in the same way as in Example 1.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.2 to 4.7 m² /V s, which were close to the valuesobtained in Example 3. Furthermore, the adhesiveness between thesuccessive layers was good similarly to Example 1. The fabricatedHall-effect devices had quite high reliability in the same manner as inExample 1.

It is to be understood that the conditions under which the buffer layer3 is fabricated are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation of the buffer layer 3.

The buffer layer 3 was epitaxially grown in the initial stage of thegrowth in the same way as in Example 1. However, observation by RHEEDhas revealed that as the Sb/In increased, the film became Sb-excessiveand film changed from an epitaxial film to an amorphous film. That is,in this method, a buffer layer 3 similar to the buffer layer 3 ofExample 3 can be formed. Then, the temperature was elevated in the sameway as in Example 3, whereby the quality of the buffer layer 3 could beimproved. The initial value of the Sb/In is preferably less than 6 asdescribed in Example 1. In order to amorphize the film within thistemperature range, it is necessary to set the Sb/In larger than 8. Morepreferably, the Sb/In is set larger than 10. In this way, semiconductorthin films 4 having high electron mobilities can be obtained undervarious conditions. In the present example, during growth of the bufferlayer, the Sb/In was increased continuously. The Sb/In may be increasedin a stepwise fashion. Also in the present example, the initial layer 2and the buffer layer 3 are formed separately. These layers may be formedsuccessively.

EXAMPLE 5

The manufacturing steps of the present example are similar to the stepsof Example 2 except for the method of forming the initial layer 2 andfor the conditions under which the buffer layer 3 is formed. Thetemperature profile of the present example is similar to the temperatureprofile of FIG. 8 but different temperatures are used.

The hydrogen termination of the substrate and introduction into vacuumequipment in Example 2 were effected. Under this condition, thesubstrate temperature was set to 380° C. Then, the initial layer 2 of Alwas formed to a thickness of 0.2 nm by electron beam evaporation. Atthis time, the evaporation rate was 0.05 nm/s. At this temperature, thebuffer layer 3 was started to be grown by coevaporation of In and Sb,utilizing resistance heating. Immediately thereafter, the substratetemperature was started to be elevated. The temperature was elevated ata constant rate such that the substrate temperature reached 430° C. atthe end of the growth of the buffer layer 3 . The Sb/In was maintainedconstant. At this time, the other conditions were the same as used inExample 2. Thereafter, the semiconductor film 4 was formed in the sameway as in Example 2.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.1 to 4.8 m² /V s, which were excellent valuesclose to the values obtained in Example 3. Furthermore, the adhesivenessbetween the successive layers was good similarly to Example 1. Thefabricated Hall-effect devices had quite high reliability in the samemanner as in Example 1.

It is to be understood that the conditions under which the initial layer2 and buffer layer 3 are formed are not limited to the foregoing. Thereason will be described below in greater detail in connection with theprocess of formation.

Where the initial layer 2 is made of Al, the crystallinity of the layeris affected greatly by the thickness of the initial layer 2 and also bythe temperature at which the buffer layer 3 is started to be grown, inthe same way as in Example 1 where the initial layer 2 is made of In.Experiments were effected on Al similarly to the experiments on In shownin FIG. 5. The thickness of the initial layer 2 on which the bufferlayer 3 was epitaxially grown was 0.1 to 3 nm. The temperature at whichthe layer was started to be grown was in the range from 250° to 430° C.

As described above, semiconductor thin films 4 having high electronmobilities can be fabricated under various conditions. Where thetemperature at which the buffer layer was started to be epitaxiallygrown is comparatively high within the above-described range, thesurface of the buffer layer 3 is roughened. Therefore, the startingtemperature is preferably below 400° C. Where the semiconductor thinfilm 4 is formed above 400° C., good film quality is obtained.Therefore, it is desired to elevate the temperature after the bufferlayer 3 is started to be formed at a low temperature.

In the present example, the buffer layer 3 was formed while elevatingthe temperature in the same way as in Example 2. However, this exampleis not restricted to this method. For example, if the temperature atwhich the buffer layer 3 is started to be grown lies within the rangefrom 250° to 370° C., then the buffer layer 3 becomes Sb-excessive.Consequently, any of the methods of Examples 1-4 can be applied. If thetemperature is between 370° to 400° C., then a stoichiometriccomposition can be easily obtained from the beginning of the growth ofthe layer. This fabricates the manufacturing steps of Examples 1 and 2,producing desirable results.

EXAMPLE 6

The manufacturing steps of the present example are similar to the stepsof Example 5 except for the manner in which the initial layer 2 isformed. In Example 5, the initial layer 2 is made of Al. In the presentexample, the initial layer 2 is made of Ga.

The temperature of a substrate was set to 380° C. after the substratewas introduced into vacuum equipment, in the same way as in Example 5.Then, an initial layer 2 made of Ga was formed to a thickness of 0.2 nmby electron beam evaporation. At this time, the evaporation rate was0.05 nm/s. Subsequently, a buffer layer 3 and a semiconductor thin film4 were formed in the same way as in Example 5.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.1 to 4.7 m² /V s, which were close to the valuesobtained in Example 5. Furthermore, the adhesiveness between thesuccessive layers was good similarly to Example 1. The fabricatedHall-effect devices had quite high reliability in the same manner as inExample 1.

It is to be understood that the conditions under which the initial layer2 is formed are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation.

Experiments were effected on Ga similarly to the experiments on In shownin FIG. 5, in the same way as in Example 5. The thickness of the initiallayer 2 on which the buffer layer 3 was epitaxially grown was 0.1 to 2nm, in the same way as in the case of In. However, the temperature rangediffered, and an epitaxial film was obtained in the range from 250° to400° C.

As described above, semiconductor thin films 4 having high electronmobilities could be fabricated under various conditions described above.If the temperature at which the buffer layer is started to beepitaxially grown is above 380° C. within this range, the surface of thebuffer layer 3 is roughened in the same way as in Example 5. Therefore,it is desired to elevate the temperature after the buffer layer 3 isstarted to be formed at a low. temperature. In the present example, thebuffer layer 3 is grown while elevating the temperature in the same wayas in Example 2 but the present example is not limited to this methodsimilarly to Example 5.

EXAMPLE 7

The manufacturing steps of the present example are similar to the stepsof Example 5 except for the manner in which the initial layer 2 isformed. In Example 5, the initial layer 2 is made of Al . In the presentexample, the initial layer 2 is made of a mixture of Al and In.

The temperature of a substrate was set to 380° C. after the substratewas introduced into vacuum equipment, in the same way as in Example 5.Then, Al was evaporated by electron beam evaporation techniques. At thesame time, In was evaporated by resistance heating to form the initiallayer 2. At this time, the Al/In was 3/2, and the total evaporation ratewas 0.05 nm/s for Al and In. The evaporation operation was carried outfor 4 seconds. Subsequently, a buffer layer 3 and a semiconductor thinfilm 4 were formed in the same way as in Example 5.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.3 to 4.9 m² /V-s, which were comparable orsuperior to the values obtained in Example 5. Furthermore, theadhesiveness between the successive layers was good similarly toExample 1. The fabricated Hall-effect devices had quite high reliabilityin the same manner as in Example 1.

It is to be understood that the conditions under which the initial layer2 is formed are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation.

As described already in Examples 1 and 5, the relation between thethickness of the initial layer 2 on which a film is epitaxially grownand the temperature at which the buffer layer 3 is started to be growndiffers between In and Al. In the case of In, the temperature is250°-350° C. at thicknesses of 0.1 to 2 nm. In the case of Al, thetemperature is 250° to 430° C. at thicknesses of 0.1 to 3 nm. In thecase of Ga, the temperature is 250°-400° C. at thicknesses of 0.1 to 2nm as already described in Example 6. Thus, the lower limit of theinitial layer and the lower limit of the temperature at which the bufferlayer was started to be grown were the same for all the cases and didnot differ where the materials were mixed. The upper limits differedamong these cases but if the materials are mixed, the upper limits maybe set to values determined from the mixture ratio by a simpleproportional distribution. In this way, the ratio of mixture can be setat will by controlling the upper and lower limits.

However, if the film is started to be grown at a relatively hightemperature within this temperature range, the surface of the bufferlayer 3 will be roughened. Therefore, as described already in Examples 6and 7, it is desired to elevate the temperature after the buffer layer 3is started to be formed at a low temperature.

As described above, semiconductor thin films 4 having high electronmobilities could be fabricated under various conditions described above.In the present example, the buffer layer 3 is grown while elevating thetemperature in the same way as in Example 2 but the present example isnot limited to this method similarly to Example 5.

EXAMPLE 8

The manufacturing steps of the present example are similar to the stepsof Example 7 except for the manner in which the buffer layer 3 isformed. In Example 7, the layer is made of InSb. In the present example,the composition was shifted from AlInSb to InSb.

An initial layer 2 was formed at a substrate temperature of 380° C. outof Al and In such that the Al:In was 3:2, in the same way as in Example7. Coevaporation of Al, In, and Sb was started by making use of electronbeam evaporation and resistance heating. At this time, the evaporationrate was 0.06 nm/s for Al and 0.04 nm/s for In. The evaporation rate ofSb was maintained at such a value that the ratio of the total number ofevaporated Al and In particles to the number of evaporated Sb particlesis 2. Under this condition, the substrate temperature was elevated inthe same way as in Example 7. The evaporation rate of Al was lowered by0.01 nm/s and the evaporation rate of In was increased by 0.01 nm/severy 20 seconds while continuing the evaporation process. After a lapseof 120 seconds, only In and Sb were evaporated. The evaporation time,the substrate temperature, and the pressure were the same as those usedin Example 7. Subsequently, the semiconductor thin film 4 was formed inthe same way as in Example 7.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.5 to 5.0 m² /V-s, which were comparable to thevalues obtained in Example 3. Furthermore, the adhesiveness between thesuccessive layers was good similarly to Example 1. The fabricatedHall-effect devices had quite high reliability in the same manner as inExample 1.

It is to be understood that the conditions under which the initial layer2 is formed are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation.

Where a film of InSb is formed on the initial layer containing Al, afilm of AlSb is formed at the boundary. Since the composition changessuddenly at this boundary, misfit dislocations occur, often leading to adeterioration in the crystallinity. In order to prevent this, it isnecessary to change the composition mildly. In the present example,therefore, the proportion of Al was reduced gradually. In this method,the lattice constant can be varied gradually from AlInSb to InSb. Asemiconductor thin film 4 having good crystallinity and a flat surfacecan be obtained. The same method is adopted for the case of the initiallayer 2 made only of Al and for the case of Ga. A semiconductor thinfilm 4 of good quality can be derived by gradually changing thecomposition from the composition of In:Al:Ga of the initial layer 2 tothe composition of the semiconductor layer 4.

As described above, semiconductor thin films 4 having high electronmobilities could be fabricated under various conditions described above.In the present example, the buffer layer 3 iS grown while elevating thetemperature but the present example is not limited to this methodsimilarly to Example 5. In the present example, the ratio of Al to Inwas changed in a stepwise fashion. It may also be varied continuously.

EXAMPLE 9

In order to increase the sensitivity of a magnetoresistor shown in FIG.4(a), it is desired to increase the electron mobility. It is known thata method of adding InBi increases the electron mobility of InSbdescribed thus far (Amemiya et al., Trans. I.E.E.J., 93-C, No. 12,(1973), pp. 273-280). In the present example, therefore, the compositionof the semiconductor thin film 4 was changed, based on the method ofExample 8.

The method of Example 8 was carried out until the buffer layer 3 wasformed. Then, the semiconductor thin film 4 was started to be formed bycoevaporation of In, Sb, and Bi, utilizing resistance heating. At thistime, In and Sb were evaporated under the same conditions. Theevaporation rate of Bi was so determined that Bi/In was maintained at0.02. The evaporation time, the substrate temperature, and the pressurewere the same as those used in Example 8.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1. The electron mobilities atroom temperature were 4.8 to 5.4 m² /V-s, which were superior to thevalues obtained in Example 8. Furthermore, the adhesiveness between thesuccessive layers was good similarly to Example 1. The fabricatedHall-effect devices had quite high reliability in the same manner as inExample 1.

It is to be understood that the conditions under which the initial layer2 is formed are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation.

Since the vapor pressure of Bi is lower than the vapor pressure of Sb byabout 2 orders of magnitude, Bi does not evaporate again from the filmwithin the range of temperatures at which the semiconductor thin film 4is formed. Therefore, a desired composition InSb_(1-x) Bi_(x) isobtained irrespective of the proportion of Sb, by maintaining the In/Biat a constant value of 1/x. That is, the proportion of Sb, theevaporation rate, and other factors can be set at will merely by settingIn/Bi at a desired value. Although the electron mobility can be improvedby adding InBi, if the percentage is in excess of 2.6%, then thecrystalline structure of InSb changes, thus deteriorating thecharacteristics. Consequently, the percentage of InBi should be lessthan this value.

As described above, semiconductor thin films 4 having high electronmobilities could be fabricated under various conditions described above.In the present example, the buffer layer 3 is grown while elevating thetemperature in the same way as in Example 2 but the present example isnot limited to this method similarly to Example 5.

EXAMPLE 10

In order to increase the sensitivity of a Hall generator shown in FIG.4(b), the Hall coefficient must be larger than the electron mobility. Itis known that the Hall coefficient of InSb is increased by adding GaSb(J. Appl. Phys. Vol. 44, No. 4, 1973, pp. 1625-1630). In the presentexample, the composition of the semiconductor thin film 4 was varied,based on the method of Example 8 in which the initial layer 2 and thebuffer layer 3 contain Al. In the present example, these two layerscontain Ga instead of Al.

An initial layer 2 was formed-out of Ga and In such that the Ga:In was3:2, in the same way as in Example 8. The manufacturing conditions weresimilar to the conditions of Example 8 except that Al was replaced byGa. Then, coevaporation of the three elements was started to form thebuffer layer 3, in the same way as in Example 8. The evaporation ratewas changed by 0.01 nm/s every 20 seconds. Ga:In was varied from 3:2 to1:9. After a lapse of 100 seconds, the ratio was maintained at 1:9.During the growth of the buffer layer 3, the substrate temperature andthe percentage of Sb were the same as those used in Example 8.Thereafter, the same manufacturing steps were effected to form thesemiconductor thin film 4. That is, Ga:In was set to 1:9. Theevaporation rate of Sb was maintained at such a value that the ratio ofthe total number of evaporated Ga and In particles to the number ofevaporated Sb particles is 2.

It was confirmed that the semiconductor thin film 4 was grownepitaxially in the same way as in Example 1 . The Hall coefficient ofInSb film of Example 8 at room temperature was 630 cm³ /C, while theHall coefficient of the film 4 of the present example was 950 cm³ /C.Furthermore, the adhesiveness between the successive layers was goodsimilarly to Example 1. The fabricated Hall-effect devices had quitehigh reliability in the same manner as in Example 1.

It is to be understood that the conditions under which the initial layer2 is formed are not limited to the foregoing. The reason will bedescribed below in greater detail in connection with the process offormation.

Since the vapor pressure of Ga is low similarly to the vapor pressure ofIn, Ga does not evaporate again from the substrate within the range oftemperatures at which the semiconductor thin film 4 is formed.Therefore, a desired composition In_(1-x) Ga_(x) Sb is obtainedirrespective of the proportion of Sb or other conditions, by maintainingthe In/Ga at a constant value of (1-x/x).

As described above, semiconductor thin films 4 having high electronmobilities could be fabricated under various conditions described above.In the present example, the buffer layer 3 is grown while elevating thetemperature in the same way as in Example 2 but the present example isnot limited to this method similarly to Example 5.

In the examples described thus far, the semiconductor thin film 4consists mainly of InSb. A mixed crystal between a single substance ofthis InSb and InBi or GaSb is used. An epitaxially grown semiconductorthin film 4 having good crystallinity can also be obtained by using amixed crystal with indium phosphide or indium arsenide.

The substrate is made of a single crystal of Si. A substrate having asurface consisting of a single crystal of Si such as a substrate of theSOI (Si on insulator) structure formed by lamination or ion implantationmay also be used.

In the examples described thus far, the initial layer 2, the bufferlayer 3, and the semiconductor thin film 4 are formed by vacuumevaporation techniques. If plasma-assisted deposition, ionized-clusterbeam deposition, or other method of forming a film utilizing the energyof appropriate plasma, ions, or the like is used, the growth temperaturecan be lowered further. Furthermore, a semiconductor thin film havinggood characteristics can be fabricated.

What is claimed is:
 1. A method of fabricating a semiconductor thinfilm, comprising the steps of:preparing a substrate having a surfaceincluding a single crystal of Si; removing an oxide film from thesurface of the substrate and terminating dangling bonds of Si atoms onthe surface with hydrogen atoms; forming an initial layer of at leastone member selected from the group consisting of Al, Ga, and In on thesubstrate terminated with the hydrogen atoms; forming a buffer layercontaining at least In and Sb on the initial layer; and forming asemiconductor thin film containing at least In and Sb on said bufferlayer at a temperature higher than a temperature at which said bufferlayer is started to be formed.
 2. The method in accordance with claim 1,wherein(A) said initial layer is formed of Al to a thickness of 0.1 to 3nm; (B) said buffer layer is started to be formed at a temperature of250°-430° C.; and (C) said semiconductor thin film is formed at atemperature of 370°-460° C. higher than said temperature at which saidbuffer layer is started to be formed.
 3. The method in accordance withclaim 1, wherein(A) said initial layer is formed of Ga to a thickness of0.1 to 2 nm; (B) said buffer layer is started to be formed at atemperature of 250°-400° C.; and (C) said semiconductor thin film isformed at a temperature of 370°-460° C. higher than said temperature atwhich said buffer layer is started to be formed.
 4. The method inaccordance with claim 1, wherein(A) said initial layer is formed out ofIn to a thickness of 0.1 to 2 nm; (B) said buffer layer is started to beformed at a temperature of 250°-350° C.; and (C) said semiconductor thinfilm is formed at a temperature of 370°-460° C.
 5. The method inaccordance with claim 1, whereinsaid buffer layer consists of In and Sb.6. The method in accordance with claim 1, whereinsaid buffer layerincludes Al, In, and Sb, and wherein said Al has a proportion decreasingwith increasing thickness of said buffer layer.
 7. The method inaccordance with claim 1, whereinsaid buffer layer includes Ga, In, andSb.
 8. The method in accordance with claim 1, whereina proportion of Al,In, and Sb of the buffer layer varies continuously or in a stepwisefashion from an interface with said initial layer to an interface withsaid semiconductor thin film, from a proportion of those of the initiallayer to a proportion of those of the semiconductor thin film.
 9. Themethod in accordance with claim 1, whereinsaid buffer layer is formedwhile maintaining said temperature at which said buffer layer is startedto be formed.
 10. The method in accordance with claim 7, whereinsaidbuffer layer is formed in such a way that a ratio of a number ofevaporated Sb particles to a number of evaporated In particles isincreased with increasing thickness of said buffer layer and then thetemperature of said substrate is elevated at least to 370° C. at a rateexceeding 0.5° C./sec.
 11. The method in accordance with claim 1,whereinsaid buffer layer is formed in such a way that the temperature ofsaid substrate is elevated with increasing thickness of said bufferlayer.
 12. The method in accordance with claim 1, whereinsaid bufferlayer is formed in such a way that the temperature of said substrate isdescended with increasing thickness of the buffer layer and then thesubstrate is elevated at least to 370° C. at a rate exceeding 0.5°C./sec.
 13. The method in accordance with claim 1, whereinsaidsemiconductor thin film consists only of indium antimonide or consistsof a mixed crystal between indium antimonide and at least one memberselected from the group consisting of indium phosphide, indium arsenide,indium bismuthide, and Ga antimonide.
 14. The method in accordance withclaim 1, further including the step of attaching electrodes to saidsemiconductor thin film.
 15. The method in accordance with claim 1,further including the step of attaching electrodes to said semiconductorthin film so as to produce a hall-effect device.